after delete trigger behavior

Hello,

I have created a trigger function to update the sort_order column of a 
mapping table. I have table a that has a many to many relation ship with 
table b that is mapped as a_b where a_id, and b_id are the pk columns and 
there is a sort_order column.  Since a_b is a mapping table there are 
foreign key constraints with a cascade option. So, if i delete an entry from 
b, an entry in a_b is deleted. What I want though is for the sort_order 
column to be updated so that all entries of a_b for a given a entry remain 
in order.

a_id, b_id, sort_order
1, 2, 0
1, 3, 1
1, 4, 2
1, 7, 3

if I delete b_id = 4 then the b_id 7 should get a sort order of 2. I created 
an after delete trigger and the trigger works just fine when i delete only 
one row, but if I delete all using "delete from a_b" I am only able to 
delete one row. Here is an example:
-----------------------------
-- a test table
CREATE TABLE test1 (
a int,
b int,
c int);
-----------------------------
-- a resort function
CREATE OR REPLACE FUNCTION resort_test1() RETURNS TRIGGER AS '
DECLARE  eachrow RECORD;  innerrow RECORD;  sort INT := 0;
BEGIN  EXECUTE ''UPDATE portfolio.test1 set c = c - 1 where a = '' || OLD.a || 
'' and c > '' || OLD.c;  RETURN OLD;
END;
' language 'plpgsql';
---------------------------------
-- the trigger
CREATE TRIGGER u_test1 BEFORE DELETE ON portfolio.test1 FOR EACH ROW EXECUTE 
PROCEDURE resort_test1();
------------------------------------------
-- dummy data
insert into test1 values(1,1,0);
insert into test1 values(1,2,1);
insert into test1 values(1,3,2);
insert into test1 values(1,4,3);
insert into test1 values(1,5,4);

insert into test1 values(2,1,0);
insert into test1 values(2,2,1);
insert into test1 values(2,3,2);
insert into test1 values(2,4,3);
insert into test1 values(2,5,4);

-- delete that works
delete from test1 where b = 3;
-- review results
select c from test1 where a = 1 order by c;
-- delete all
delete from test1;

---- note that it will only delete one row.

Is this by design? Is there something I can do to remedy this behavior? I 
would expect to have all rows delete and not just the first one.

Any help is appreciated.

Russ